The practice of selectably coupling a signal in either DC or AC modes is important in such fields as electrical testing apparatus. A basic AC or DC coupling selection circuit includes a capacitor connected in parallel with a conventional SPST switch. When the switch is open, AC current flows through the capacitor while DC current flow is blocked by the capacitor. Conversely, when the switch is closed, both DC and AC current flow through the switch, bypassing the capacitor.
However, simple circuit embodiments of a selectable AC/DC coupler are impractical in most high frequency environments. Typical implementation of the simple capacitor-switch circuit has been attained by utilizing commercially available components and hooking them together conventionally by using printed circuit boards or other modular techniques. Such circuits are often unwieldy, and suffer high insertion losses when utilized within precision environments requiring shielded transmission lines such as coaxial cables. In particular, the transition from signal source to coupler and from coupler to a reception point of interest invariably creates impedance mismatching which can give rise to significant standing wave problems. Also, variations in the positions of switches can affect the electrical properties of the coupler. Movement of the switch has an affect on the field thus altering the characteristic impedance of the coupler.